1. Field of the Invention
The present invention relates to a liquid crystal display panel and a method of manufacturing the same.
2. Description of the Related Art
FIG. 1 is a schematic sectional view showing the structure in the vicinity of the frame of the liquid crystal display panel of a related art. Liquid crystal display panel 1 has TFT substrate 2 on which switching elements 8 such as thin film transistors (TFTs), for example, are formed in a matrix shape and CF substrate 3 on which color filter (CF) 7, black matrix (BM) 6 and the like are formed. Alignment layers 9 to which alignment treatment is applied, are formed respectively on the opposed surfaces, which are opposite to each other, of substrates 2 and 3. In order to form a predetermined gap between both substrates 2 and 3, pillar spacer 10 and seal material 5 are formed between both substrates 2 and 3. Liquid crystal material 4 is sealed in the gap by them.
FIG. 2 is a flowchart showing a method of manufacturing the liquid crystal display panel shown in FIG. 1. First, in the seal coating step, a seal material is coated onto a predetermined position of the TFT substrate, and an outer peripheral (auxiliary) seal and a main seal are formed in an uncured state.
Subsequently, in the Ag coating step, Ag-transfer is coated to a predetermined position of the TFT substrate in a dotted form (in the case of manufacturing the liquid crystal display panel of a Twisted Nematic (TN) mode).
Next, in the liquid crystal dropping state, a liquid crystal material is dropped by a predetermined drop amount to a predetermined position inside the above described main seal in a matrix form, a linear form or a radial form. In the pixel region of the CF substrate, a pillar spacer or a spherical spacer for forming a predetermined gap between the substrates is disposed in advance.
Thereafter, in the bonding step, both the above described substrates are brought into contact with each other and pressurized. Thereby, the liquid crystal material is uniformly diffused into the entire pixel region between the substrates, and the gap between the substrates is uniformly formed.
When the substrates bonded to each other are transported for the next step, temporary fastening is performed for suppressing misalignment in fitting both the substrates. As temporary fastening of the seal material, ultraviolet (UV) temporary curing is partially performed for several spots. At this time, the liquid crystal material quickly diffuses with the lapse of time, and passes from the pixel region to the frame region (region where BM is formed) outside of the pixel region and reaches the seal material. Specifically, then, the liquid crystal material is in contact with the seal material.
Next, in the UV curing step, the seal material is cured by UV radiation. At this time, the seal material is in a semi-cured state. Until the UV curing step is completed, the seal material in the uncured state and the liquid crystal material are in contact with each other. Further, in the heat curing step which is performed next, the seal material is completely cured, but at the early stage of the heat curing step, the seal material in the semi-cured state and the liquid crystal material continue to be in contact with each other.
As described above, in the liquid crystal panel shown in FIG. 1, the seal material in the uncured or semi-cured state and the liquid crystal material are in contact with each other during the bonding step, the UV curing step and the heat curing step. Thereby, as shown in FIG. 1, a large quantity of impurities 16 such as, for example, impurity ions elute from seal material 5 into liquid crystal material 4. Thus, the liquid crystal material 4 is contaminated by impurity ions.
As a substance which contaminates liquid crystal material 4, there are cited an oligomer component, an organic matter such as phthalate ester, ion impurities such as Na, K and Cl and the like which elute from seal material 5 in the uncured or semi-cured state.
Further, in the above described liquid crystal panel, liquid crystal material 4 is in contact with seal material 5 in the uncured or semi-cured state, and liquid crystal material 4 sometimes erodes the interface of seal material 5 and the base of seal material 5. According to this, the problem of reduction in seal bonding strength in the interface between seal material 5 and the substrate occurs. As a result, a stain, unevenness, seal removal and the like sometimes occur in the peripheral portion of the seal material of the liquid crystal display panel after a reliability test is performed. Hence, in the above described liquid crystal panel, reduction in display quality and in reliability occur.
As related art that describes solving the problems of contamination of the liquid crystal material and reduction in the bonding strength of the sealing material due to contact of the seal material in the uncured or semi-cured state with the liquid crystal material, the following two related arts can be cited.
(1) Related Art Using a Barrier
(2) Related Art of Increasing the Surface Energy
FIG. 3 shows one example of a liquid crystal display panel using art (1) using a barrier. In the liquid crystal display panel, projected portion 12 is formed between the pixel region and the region coated with seal material 5 of TFT substrate 2 as shown in FIG. 3. Diffusion of liquid crystal material 4 is suppressed by adjusting the distance between projected portion 12 and CF substrate 3 which is opposite to projected portion 12.
As documents describing related art using the barrier, there are cited Japanese Patent Laid-Open No. 1999-38424, and Japanese Patent Laid-Open No. 2003-315810. In the liquid crystal display panel described in Japanese Patent Laid-Open No. 1999-38424, projected portions 12 are formed between the pixel region and the region coated with seal material 5 of the TFT substrate. It is disclosed that a vertical alignment layer is formed on the top portion of projected portion 12 as an alignment layer. Japanese Patent Laid-Open No. 2003-315810 discloses that a number of flow control walls 13 for liquid crystal material 4 are formed in the pixel region, or inside and outside the pixel region, as shown in FIG. 4.
Related art of increasing the surface energy of the above described (2) is disclosed in Japanese Patent Laid-Open No. 1998-260406. In the publication, the surface of the alignment layer at the position away from the display region which is the pixel region is reformed. Thus, the surface energy of the reformed surface is increased more than that of the alignment layer of the display region.
In Japanese Patent Laid-Open No. 1999-38424, liquid crystal diffusion control is performed using the vertical alignment layer and the projected portion, and the effect can be obtained only in the liquid crystal display panel using the vertical alignment layer that has low wettability to the liquid crystal material. Accordingly, there is the problem that in the case of using the other ordinary alignment layers, liquid crystal diffusion speed cannot be decreased.
Further, the projected portion is formed so that the clearance between the projected portion and the substrate opposite to the projected portion becomes extremely small, and therefore, the problem of contamination of the liquid crystal material is solved to a certain extent. However, no consideration is given to the fact that the gap in the peripheral portion of the seal material becomes uneven by forming the projected portion. Specifically, the suppressing method of the liquid crystal diffusion according to the above described publication directly stops the flow of the liquid crystal material, and the diffusion speed is controlled by the height or the like of the projected portion. Therefore, in order to decrease the liquid crystal diffusion speed, a high projected portion needs to be formed in a wide range in the peripheral portion of the seal material.
Depending on the variation of the height of the projected portion, there arises the problem that the projected portion locally contacts the substrate that is opposite to the projected portion. When the projected portion and the substrate are in contact with each other, local stress occurs, and a variation of gap between substrates occurs in the peripheral portion of the seal material. As a result, reduction in display quality occurs in the boundary of the black matrix formation part and the pixel region. This reduction becomes significantly noticeable as the distance between the substrates becomes shorter.
Further, in Japanese Patent Laid-Open No. 2003-315810, a number of flow control walls each in the shape of the projected portion for directly stopping the flow of the liquid crystal material are disposed. Therefore, the problem of contamination of the liquid crystal material is solved to some extent. However, consideration is not given to unevenness of the gap which occurs as a result of disposing a number of the above described flow control walls. Specifically, the suppressing method according to the above described publication suppresses liquid crystal diffusion by disposing a number of flow control walls each in the shape of the projected portion for directly stopping the flow of the liquid crystal material.
A number of the flow control walls are disposed in the pixel region, or in the pixel region and outside the pixel region so as to be in contact with the substrate that is opposite to the above described flow control walls. Therefore, in the pixel region and the peripheral portion of the seal material, variation of the gap due to contact stress occurs. As a result, display quality of the pixel region degrades, and display quality in the boundary of the black matrix formation part and the pixel region degrades.
Further, in Japanese Patent Laid-Open No. 1998-260406, by increasing the surface energy of the alignment layer at the position away from the display region more than that of the alignment layer of the display region, the problem in which the ionic impurities in the seal material diffuse to the liquid crystal material is solved to some extent. However, the fact that the seal material in the uncured or semi-cured state is in contact with the liquid crystal material does not change. Specifically, in the liquid crystal panel described in the above described publication, the surface energy of the alignment layer at the position away from the display region is increased more than that of the alignment layer of the display region, and as a result, the ionic impurities which elute or which have already eluted to the liquid crystal material from the seal material are only adsorbed by it and suppressed from diffusing to the liquid crystal material. Accordingly, the seal material in the uncured or semi-cured state and the liquid crystal material are in contact with each other from the manufacturing stage. Accordingly, a large quantity of ion impurities cannot be suppressed from eluting from the manufacturing stage.
Diffusion of the liquid crystal material is not controlled in the bonding step, the UV curing step or the heat curing step as described above, and therefore, the seal material in the uncured or semi-cured state and the liquid crystal material are in contact with each other. As a result, the problems of contamination of the liquid crystal material and reduction in the seal bonding strength occur.